Precision wire grid glass digitizing tablets

ABSTRACT

An improved wire grid glass tablet for digitizing graphic information which includes a glass plate with a resin layer bonded to one side and having orthogonal grids of wires, made of a ductile material which is stretched beyond the yield point, embedded in the resin layer near the surface thereof to maximize signal-to-noise ratio coupling to the wires.

This is a division of application Ser. No. 06/433,602, filed Oct. 8,1982 now U.S. Pat. No. 4,513,043, issued Apr. 23, 1985.

TECHNICAL FIELD

This invention relates to an improved wire grid glass tablet, used, forexample, in digitizers for digitally encoding graphic location andmovement information into corresponding electrical signals with highaccuracy. The invention also relates to new methods and apparatus forfabricating the high accuracy wire grid glass digitizing tablets.

BACKGROUND ART AND PRIOR ART STATEMENT

Digitizer tablets are used in conjunction with a writing pen orinstrument for converting graphic information into digital coordinatesignals. The tablet or platen is formed with an XY grid of parallelconductors to which signals are applied for detection by the pen orinstrument or which are scanned to detect signals from the pen orinstrument. A variety of known methods are available for converting thelocation and movement information of the pen or instrument relative tothe tablet into electrical signals such as, for example, described inU.S. Pat. Nos. 3,767,858; 3,983,322; 4,022,971 and 4,185,165.

A typical digitizer or platen has first and second sets of gridconductors in parallel planes embedded in a resin plate having a flatwork surface referred to as the digitizing surface. The conductors ofthe first grid run perpendicular to the conductors of the second gridand all the grid conductors are electrically insulated from each other.The writing pen or other instrument bears upon the digitizing or workingsurface.

Prior tablets or platens of a resin material are typically manufacturedby pulling grid wire material such as 0.02 inch piano wire or musicwire, a structurally strong steel wire, between precision spacing guidesor bridges. The piano wire is cut to length and loops are formed at eachend. Tension is applied to each individual wire aligned over a cavity,using, for example, springs to hold them tightly. In this manner, twosets of conductors in the X and Y coordinate directions are tautlysuspended in planes spaced from each other approximately 0.05 inchesapart. The taut X and Y conductors extend through a mold or cavity and arubber gasketed cover is placed over the wire matrix. Liquid resins suchas liquid vinyl resin or catalyzed polyester resin are introduced intothe cavity or mold. After the resin has been poured and the mold orcavity filled so that the X and Y coordinate grids are immersed andembedded in the liquid, the resin is allowed to cure for approximately14 hours at a temperature of approximately 75 degrees F. The exothermtemperature of the resin at the time of curing may reach 200 degrees F.,however.

A number of disadvantages are attendant upon this conventional methodfor fabrication of digitizer tablets and platens. It is difficult toapply sufficient tension to the highly tensile piano wire to straightenout kinks or bends in the wire and the taut music wire presents a dangerto workers. It is costly to cut the wires individually to length andform loops and highly accurate positioning of the individual wires isdifficult to achieve. Furthermore, during curing the polyester resinshifts, shrinks, expands and distorts the wire matrix grid pattern. Thedisplaced grid conductors do not return precisely to their originalposition thereby limiting the accuracy of the grid conductors forencoding graphic position information.

Another method of fabricating digitizer tablets or platens uses screenprinting and printed circuit techniques for forming the grids ofparallel conductor strips on first and second glass plates. As describedin U.S. Pat. No. 4,255,617, the X and Y coordinate grids are formedrespectively on two pieces of float glass as shown in FIG. 3 of thatpatent, and the two pieces of glass are laminated together by a resinlayer that separates and insulates the orthogonal X and Y gridcomponents. Such a method, however, suffers the disadvantage thatmultiple glass plates must be used, and the printed circuit traces aresubject to damage or defect during manufacture in contrast with the wiregrid matrix.

U.S. Pat. No. 2,194,551 describes a method for producing a polarizingbody consisting of a glass plate with fine wires embedded in the glassto a density of, for example, 40,000 or more parallel layers per inch.According to the disclosure in this patent, a method is proposed forpacking powdered glass above and below the mounted wires and heating thewhole mass in a furnace so that the glass and wires are heatedsimultaneously, the glass melting around the wires. The glass and wireare then stretched together while the glass is in a plastic state. Thestretching elongates the glass and wires, bringing the wires closertogether for light polarizing effects. This U.S. patent disclosure isfor an entirely different purpose from the present invention, actuallyheating and stretching both a glass material and embedded set of wirestogether.

U.S. Pat. No. 2,194,551 relates only to methods of producing flatsurfaces such as mesh screens or screen electrodes by a stretchingtreatment and is otherwise unrelated to the present invention.

OBJECTS OF THE INVENTION

It is therefore an object of the present invention to provide adigitizer tablet or platen using separate wire grids for the X and Ycoordinate matrix, but based upon a glass plate substrate for stabilityand freedom from the parametric variation characteristics of plastic andresin tablets.

Another object of the invention is to provide a glass tablet or platenwith highly accurate wire location, and straightness of the wirescomposing the X, Y coordinate grid.

Another object of the invention is to provide a wire grid glass platenusing ductile wire rather than wire having a relatively high yield pointfor high accuracy and straightness of the wire by application of tensionbeyond the yield point of the ductile wire.

A further object of the invention is to provide a wire grid glass platenwherein the wire grids are close to the surface thereof to maximizesignal-to-noise ratio for a pen/cursor.

Finally, the invention contemplates a new method for manufacture of highaccuracy wire grid tablets and platens using a single glass plate and toa fixture and framework for carrying out the method.

DISCLOSURE OF THE INVENTION

In order to accomplish these results the present invention provides animproved wire grid glass tablet for digitizing graphic information inthe form of corresponding electrical signals in which a flat glass plateforms the structural base for the tablet. A resin layer is bonded to onesurface of the glass plate.

According to the invention, a first grid of accurately spaced parallelwires is embedded in the resin layer and arranged in a common planeparallel to the plane of the glass plate. In particular, the wirescomprise a ductile material having been stretched to beyond the yieldpoint, thereby exhibiting plastic flow and straightness. A second gridof parallel wires is also immersed in the resin layer spaced from thefirst grid, the first and second grids forming the X, Y coordinatematrix. The second grid also comprises accurately spaced parallel wiresin a common plane parallel to the plane of the glass plate, but ofcourse, spaced from and insulated from the first grid. The wires of thesecond grid have also been stretched to beyond the yield pointexhibiting plastic flow and straightness.

According to another feature of the invention the resin layer isrelatively thin with respect to the glass. The resin remains in thisthin uniform state during curing because it is placed on a levelledglass plate, because of its viscosity, and because of the forces ofgravity and surface tension.

It is apparent that the tablet or platen construction according to thepresent invention affords the stability and accuracy of a glass platestructural base while avoiding the more expensive and vulnerable printedcircuit X, Y coordinate matrices. The invention retains the advantagesof a wire grid but minimizes the plastic or resin layer, assures thestraightness of the wires comprising the matrix, and a safe environmentfor those assembling the wires.

According to the method of fabricating the precision wire grid glasstablet, a first wire grid is formed by stringing wire of ductilematerial on a frame assembly around accurately aligned and spacedrollers, traversing the lengths of wire back and forth in a parallelarray. A second grid of wires is similarly formed on the frame assemblyin a second parallel array spaced from the first, with the lengths ofwire running perpendicular to the lengths of wire of the first grid.

The invention further contemplates preparing a flat glass plate byspreading a resin material over the flat upper surface of the glassplate and delaying curing of the resin layer during a period of timepermitting the resin layer to self-distribute evenly under the influenceof gravity and surface tension. This may take, for example, 15-30minutes for settling after pouring or manually leveling the resin layer.

An important step of the invention is tensioning the lengths of wire ofthe first and second grids to beyond the yield point of the ductilematerial comprising the wire until the wire exhibits plastic flowthereby assuring straightness of the lengths of wire.

In another critical step, the glass plate is raised beneath the firstand second grids, immersing the lengths of wire of the grids in theresin layer supported on the upper surface of the glass plate. Afterseveral minutes' settling, the resin layer is rapidly cured, embeddingthe X,Y coordinate grid and bonding to the glass plate. Rapid curing isachieved using anaerobic and ultra-violet light sensitive resins whichundergo rapid curing upon flooding, for example, with nitrogen or carbondioxide to exclude oxygen and upon irradiating with an ultra-violetlight fixture operatively positioned relative to the glass tablet.

The invention contemplates a number of variations in the method offabricating high accuracy wire grid glass tablets and supplementalsteps. For example, during the period of settling of the resin layer onthe flat and level glass plate, a volatile solvent may be sprayed overthe surface of the resin layer prior to curing thereby releasing bubblesfrom the surface to facilitate leveling and settling. In a more detailedlook at the method, each wire grid of the respective X or Y coordinatematrix comprises a single wire assembled by stringing a single piece ofwire back and forth in serpentine fashion around rollers at oppositesides of the parallel array. In a further subtlety of the method, thetensioning of the wire is accomplished by distributing the tensioningforce evenly to each of the lengths of wire of the parallel arraycomprising, for example, 320 wires of 85" segments.

In order to carry out the method the invention provides an apparatus forfabricating high precision wire grid glass tablets in the form of aheavy weight base fixture constructed and arranged for bearing wiretensioning forces. Since each wire element of the grid is subjected to,for example, 8 pounds of force for 28 gauge tinned copper wire, with asmany as 320 such wires comprising the longer grid coordinate, forcestotaling thousands of pounds may be exerted. The base structure has anupper portion for supporting and retaining a grid wire frame assembly onwhich the grid wires are mounted and a central cavity portion comprisinga lift platform for raising and lowering a plate or tablet in accuratelyaligned horizontal position beneath the X,Y coordinate wire grids.

The base fixture is a four-sided framework, each side having an outerbearing surface for receiving the grid wire frame assembly and an innerwire locating rail for accurately spacing and aligning the wires of anX,Y coordinate grid in the manner hereafter described.

The grid wire frame assembly is clamped to the outer bearing surfaces ofthe base fixture four-sided framework and at least two sides of the gridwire frame assembly are separable. Two of the outer bearing surfaces ofthe base fixture four-sided framework comprise wire pull-out bars whichare also separable and moveable outward from the respective assembly andfixture for tensioning the lengths of wire of an X,Y coordinate gridbeyond the yield point of the ductile material used for the wire.Hydraulic cylinders exert the tensioning force and form an integral partof the base fixture.

The grid wire frame assembly is a four-sided framework with rollersaccurately spaced around the face of the framework. Each of the X and Ycoordinate portions of the conducting grid or matrix is actually asingle wire, winding alternately back and forth around the rollers oftwo opposite sides. Therefore, when the grid wire frame assembly isclamped to the base fixture and tension is applied to the respective Xand Y coordinates of the grid by wire pull-out bars of the base fixture,the tensioning force is distributed evenly among lengths of wire by therollers.

After tensioning, hold-down bars are mounted around the four-sidedframework between the outer bearing surfaces on which rest the sides ofthe grid wire frame assembly and the inner wire locating rails with theaccurately spaced grooves. The hold-down bars urge the wires of eachrespective coordinate into the grooves of the wire locating rail,thereby accurately spacing and aligning the lengths of wire.

The base fixture is also formed with a central cavity and a liftplatform mounted for vertical translation within the cavity. The liftplatform receives and supports a glass substrate and is raised andlowered within the cavity by, for example, hydraulic lifts.

Before the grid wire frame assembly is placed on the base fixture, theglass substrate is placed on the lift platform while it is in raisedposition. The substrate is prepared with a layer of uncured resin overits surface. After settling and flattening of the resin layer, the liftplatform is lowered and the grid wire frame assembly placed in positionover the fixture. The grid wires are then tensioned and accuratelyspaced and aligned as set forth above. The lift platform then raises theglass substrate to a stop position where the X and Y coordinates of theconductive grid or matrix are immersed in the resin layer.

After settling, an ultra-violet light hood with an area coextensive withthe base fixture and grid wire frame assembly is lowered substantiallycovering the upper surface of the base fixture and grid wire frameassembly. Rapid curing of the resin layer is achieved by ultra-violetirradiation. Furthermore, the hood serves as a manifold for deliveringan anaerobic gas, such as nitrogen or carbon dioxide for excludingoxygen. An anaerobic curing resin is preferably used for acceleratingthe cure upon displacement of the oxygen. The lift platform is also aplatform provided with stop means for stopping the raising of the glassplate and resin layer when the first and second grids supported on thewire frame assembly are immersed in the resin layer spread across theupper surface of the glass plate.

The invention also contemplates incorporating the high accuracy wiregrid glass tablet in a laminar structure or honeycomb sandwich structurefor increased strength and durability in tension and compression.According to this sandwich structure, a layer of Formica type materialor other working surface material is bonded over the resin layer of thetablet to provide the working surface, writing surface, or digitizingsurface. The glass side of the tablet is bonded to a honeycombstructural layer of, for example, phenolic impregnated paper which is,in turn, bonded to a steel sheet of 18 to 24 gauge. Such a laminar orsandwich structure gives strength in tension and compression to theglass tablet or platen. Furthermore, the steel sheet provides anelectromagnetic shield for greater accuracy in digitally encodinggraphic information.

A back-lighted laminar tablet structure may also be provided accordingto the invention, substituting a translucent or transparent material forthe Formica working and digitizing surface and a wire screen for theshield and eliminating or substituting transmitting material for otheropaque layers.

In each of the embodiments of the present invention, it is to be notedthat a glass plate forms the structural layer of the glass tablet itselfand is relatively immune from parametric variation under environmentalstress. The resin layer is a relatively thinner layer bonded to thesurface of the glass plate and merely serves the purpose of immersingand insulating, embedding and retaining the X,Y coordinate grids. Theresin layer conforms to the stable glass substrate during variation inprevailing environmental conditions.

Other objects, features and advantages of the present invention willbecome apparent in the following specification and accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary perspective view of a wire grid glass digitizingtablet in accordance with the present invention showing the relativelythick, flat glass substrate and the resin layer bonded to the glasssubstrate in which is embedded the "X, Y" coordinate grid of conductingwires.

FIG. 2 is a plan view of the glass tablet showing a fragmentary portionof the X,Y coordinate grid.

FIG. 2A is a side view of the glass tablet.

FIG. 2B is a detailed cross-sectional view of a fragmentary portion ofthe glass tablet shown in FIG. 2A.

FIG. 3 is a plan view of the glass tablet assembled and bonded into abase for containing the glass tablet electronics.

FIG. 3A is a side cross-sectional view of the tablet assembly of FIG. 3.

FIG. 3B is a detailed cross-sectional view of a fragmentary portion ofthe side cross-section of FIG. 3A showing the details of the laminartablet assembly.

FIG. 4 is a perspective view of the structural framework for the fixtureor apparatus used in fabricating the wire grid glass tablet.

FIG. 5 is a plan view from above of the glass tablet fabrication fixtureshowing the central lift platform which supports the glass substrate andthe peripheral elements mounted on the fixture used in fabricating thewire grid glass tablet.

FIG. 5A is a detailed plan view, FIG. 5B is a detailed side view, andFIG. 5C is a detailed end view of a segment of the wire locating railsshown in FIG. 5.

FIG. 5D is a detailed end view of a hold-down bar or wire clamp bar ofthe type shown in FIG. 5.

FIG. 6 is a detailed fragmentary side view of one of the two sides ofthe fixture illustrated in FIG. 5 in which the outer bearing surfacecomprises a pull-out bar with means for extending the pull-out bar tostretch lengths of wire.

FIG. 7 is a plan view of a grid wire frame assembly on which isassembled the X,Y coordinate grid of conducting wires by stringing awire back and forth around the rollers mounted along opposite sides ofthe grid wire frame assembly.

FIG. 7A is a detailed side cross-section of one of the rollers mountedaround the sides of the grid wire frame assembly of FIG. 7.

FIG. 7B is a perspective view showing the manner in which the sides ofthe grid wire frame are assembled.

FIG. 8 is a diagramatic perspective view showing the steps of tensioningwires of the X,Y coordinate grid beyond the yield point of said wiresand holding down the wires urging them into grooves of the wire locatingrail.

FIG. 8A is a detailed fragmentary view of the wire locating rail showingthe position of grid wires in the wire locating rail grooves held in thegrooves by the hold-down bar after stretching.

FIG. 9 is a side view of the hood positioned over the fabricatingfixture for ultra-violet irradiation and anaerobic gas flooding, whileFIG. 9A is an end view of the hood.

DESCRIPTION OF PREFERRED EMBODIMENTS AND BEST MODE OF THE INVENTION

The grid wire glass tablet and glass tablet assembly, according to thepresent invention, are illustrated in FIGS. 1 through 3. As shown inFIG. 1, the basic wire grid glass tablet 10 comprises a flat, glasssubstrate 12, such as, for example, 1/8" (3.2 mm) float glass and arelatively thinner resin layer 14 bonded to the surface of the glasssubstrate. The resin layer 14 is preferably a resin or resin mixturewhich rapidly cures under anaerobic conditions and in response toultra-violet radiation. Such resins include, for example, acrylic andurethane resins. Embedded in resin layer 14 is an X,Y coordinate grid ormatrix 15 of conducting wires for conducting the digitizing signals.

Reference in this specification and accompanying claims to an X,Ycoordinate grid or matrix is intended to refer to a first grid ofparallel wires 16 representing one of the X or Y coordinate directions,and a second grid of parallel wires 18 orthogonal to the first grid 16and representing the other of the X or Y coordinate directions. Each ofthe parallel grids 16 and 18 is formed in a common plane, spaced fromthe plane of the other so that all of the individual wires of theassembled grid or matrix 15 are insulated from each other within theresin layer 14 of tablet 10.

According to the invention, the conductor grid wire is made of a ductilematerial such as copper or copper alloy and in the illustrated example,is 28 gauge tinned copper wire. The wire lengths are typically spaced,for example, 0.2 inch (0.5 cm) on center for a spatial frequency of 5wires per inch (5 wires per 2.54 cm) across the tablet. The tabletstypically range in size up to 48 inches (122 cm) by 64 inches (162.5cm).

Each of the wires of the grid elements 16 and 18 has been tensionedbeyond its yield point and stretched a predetermined amount to assurestraightness of the wire prior to embedding in the resin layer. Forexample, in the large size wire grid glass tablet and in the longdirection (64 inches/162.5 cm) the original lengths of wire across thegrid wire frame during fabrication are 85 inches (216 cm) long. Theselengths of wire are each stretched, for example, four inches (10 cm) toachieve the objectives of straightness and accuracy according to theinvention.

A plan view of the glass tablet showing only a portion of the wire gridis illustrated in FIG. 2 with the corresponding reference numeralsindicated accordingly. In the side view of FIG. 2A it can be seen thatthe resin layer 14 forms a relatively thin layer, for example, 50thousands of an inch (1.3 mm) bonded to the thicker and more stableglass substrate 12. The wire grid or matrix 15 can be seen extendingfrom resin layer 14.

In the more detailed cross-sectional view of FIG. 2B it can be seen thatthe respective wire grid elements 18 and 16 are spaced from each otherwithin the resin layer 14 which is, in turn, bonded to the glasssubstrate 12.

The terminal ends 20 of parallel grid wire element 18 may terminate atone side in a common lead wire 21, while the other ends 22 from the gridelement 18 are left free for circuit connections to the electronics ofthe tablet assembly. Similarly, the terminal endings 23 at one side ofthe longer parallel wire grid element 16 may terminate in a commonleadwire 24. The opposite ends 25 of the parallel wires of grid elementsare left free for coupling to the tablet assembly electronics.

Alternatively, the grid wire terminations or ends 20 and 23 of tablet 10within the tablet assembly may be left free and uncoupled, analagous toantenna terminations. Whether or not the grid wire terminal ends 20 and23 are respectively joined to common leads or whether they are left asfree antenna terminal ends depends on whether or not the tablet isoperating with the grid in an active generating mode or passive signalreceiving mode. Both types of tablets are available according to theapplication requirements.

Referring to FIG. 2B, the width of glass substrate 12 is typically 0.12inches (0.3 cm), the width of the resin layer 14, 0.05 inches (1.3 mm),and the spacing between the center lines of the parallel wire conductorgrid elements 16 and 18, 0.025 inches (0.6 mm), while neither of thegrid elements is closer to either surface of the resin layer than 0.011inches (0.3 mm).

Referring to FIG. 3, the glass tablet 10 is typically incorporated inand bonded to a base 30 as part of the tablet assembly. The tabletassembly base 30 is made from steel sheet metal of, for example, 18 to24 gauge thickness. The tablet 10 is bonded to a layer of honeycomb 34(see FIG. 3B) which is, in turn, bonded to the bottom sheet 31 of base30, and the bottom sheet 31 therefore forms an integral laminar layer ofthe tablet assembly. Because the base is made of steel sheet, itprovides an electromagnetic shield to assure high accuracy of thedigitizing signals.

Referring at the same time to both FIGS. 3 and 3A, it is apparent thatthe tablet assembly base 30 is substantially longer along the X, Ycoordinate dimensions than the tablet 10 and the tablet is mountedtowards a corner of the base, leaving spaces 32 on two adjacent sides ofthe assembly between the tablet 10 and base 30 for housing the grid wireglass tablet electronics. Smaller spaces 33 on the other two adjacentsides between the tablet 10 and base 30 provide enough space for thecommon conductors 21 and 24 joining the terminal ends 20 and 23respectively of the grid wire conductors on the sides of the tabletadjacent the narrower spaces 33 or for leaving free "antenna" terminalendings. On the other hand, conductors 22 and 25 on the oppositeadjacent sides of tablet 10 are available for coupling to the tabletelectronics.

The electronic circuits associated with tablet 10 are not a portion ofthe present invention, and are well known and readily available.Examples of digitizer tablet electronics are found in the patents citedin the background portion of the present specification.

FIG. 3B shows a detailed side cross-section of a fragment of the tabletassembly of FIG. 3A according to one example of a completed laminarstructure of a multi-layered grid wire glass tablet. In FIG. 3B theglass substrate 12 and bonded resin layer 14 are shown within additionallaminar layers of the tablet assembly. Bonded to the exposed side ofglass substrate 12 is a layer of honeycomb 34, for example, 0.75 inches(2 cm) thick, of phenolic impregnated paper cells, for example, 3/8 inchto 1/2 inch (1 to 1.3 cm) cells. The honeycomb layer 34 is glued byadhesive to the glass substrate 12 on one side and to the steel sheet 31on the other side, strengthened by adhesive fillets which build up oneither side of the edge contact conjunctions between the honeycomb 34and the glass substrate 12 on the one hand and the steel sheet 31 on theother hand. Interposition of the adhesively bonded layer of honeycomb 34imparts strength in tension and compression to the tablet assembly.

On the resin layer side 14 of the glass tablet is adhesively bonded alayer of Formica-type material to provide a working surface layer 35,upon which working surface graphic information is expressed fordigitizing, using a digitizing pen or other instrument. Such a layer ofFormica-type material or similar material may be typically 0.031 inches(0.7 mm) in thickness.

A typical glue, adhesive or bonding material for the laminar layerswould be, for example, a urethane glue.

Before turning in detail to the method of fabricating the wire gridglass digitizing tablets, attention is directed to an apparatus andfixture for carrying out such a method. The manufacturing apparatus ishoused on a heavy duty and heavy weight framework 40 which may beanchored to a cement floor or other ground work for further stability.The fixture framework 40 is provided with heavy legs and braces as shownin FIG. 4 and an upper four-sided surface 42 around the periphery forsupporting in part a grid wire frame assembly and other apparatuselements for a glass digitizing tablet fabrication as hereafterdescribed. The upper periphery 42 of the fixture 40 defines a centralcavity 43 in which is mounted the lift platform and hydraulic lift bybraces 44a and 44b, etc. Because the fixture framework may have to bearseveral thousand pounds of force in the manner hereafter described, itis advisable to bolt the fixture to a cement floor for rigidity andaccuracy.

FIG. 5 is a plan view of the fixture with glass digitizing tabletfabrication components and elements assembled on the fixture frameworkof FIG. 4. The assembled fabricating fixture includes the flat outerbearing surfaces 42 of the fixture framework as previously shown in FIG.4, supplemented by pull-out bars 45a and 45b which effectively functionas the outer bearing surfaces 42 of the fixture framework on the twosides 42a and 42b. A grid wire frame 50 on which is strung and assembledthe X, Y coordinate grid or matrix of conductive wires is shown in FIG.5 resting in position on the top of the fixture framework 40 with twosides 50a and 50b of the grid wire frame assembly 50 resting on thebearing surfaces of pull-out bars 45a and 45b on the two sides of thefixture framework 42a and 42b. On the opposite two adjacent sides of thefixture 40 the grid wire frame 50 and in particular, sides 50c and 50drest upon the outer bearing surface sides 42c and 42d respectively.

Thus, as will hereaftermore fully appear, the grid wire frame assembly50 upon which the X,Y coordinate grid of conducting wires is assembledby stringing extends beyond the dimensions proper of fixture framework40 so that two sides of the grid wire frame assembly rest on the outerbearing surfaces 42c and 42d of the fixture framework while the othertwo sides 50a and 50b of the grid wire frame assembly extend beyond theouter bearing surface sides 42a, 42b proper of the fixture framework 40to rest upon the pull-out bars 45a and 45b respectively which functionas extendable sides of the framework as is hereafter more fullyexplained.

The sides 50a, 50b, 50c and 50d of the grid wire frame assembly 50 arealigned on the outer bearing surfaces of the fixture at the wire pullingbars 45a and 45b and bearing surfaces 42c and 42d by means ofpre-aligned dowel pins and dowel pin holes (not shown). Furthermore, thesides of frame 50 are clamped to the respective bearing surfaces by 3 to4 pressure clamps per side, not shown.

Mounted within the central opening or cavity 43 of the fixture framework40 is a lift platform 55 constructed and arranged to support the glasssubstrate of the glass digitizing tablet during the fabrication process.Lift platform 55 is mounted over hydraulic lifts (not shown) which raiseand lower upon command the platform 55 within the central cavity 43.

Mounted around the inside of the peripheral surface of the fixtureframework are four wire locating rails 56, respectively identified as56a, 56b, 56c and 56d. The wire locating rails 56 are comprised ofadjacent coupled segments shown in further detail in FIGS. 5A, 5B, and5C. Each wire locating rail 56 or wire locating rail segment is providedwith an upward projecting wall portion 57, having a flat upper surfacein which are formed the wire locating grooves 58 which serve tostraighten and align the wires during the fabrication process. Thegrooves 58 are typically formed 0.2 inches (center to center) and to adepth to accommodate the wire, for example, 28 gauge tinned copper wireor 12.6 one-thousands inch (0.3 mm) with a tolerance of plus or minus0.2 mils.

Also located around the periphery of the fixture framework, but outsidethe wire locating rails 56 are the hold-down bars 60 referred torespectively on the four sides as 60a, 60b, 60c and 60d. The hold-downbars 60 are mounted over the peripheral surfaces 42 of the fixture bymeans of threaded clamps 61 with handles for turning the mountingthreads to raise and lower the hold-down bars 60. The threaded clamps 61permit application of downward force on the hold-down bars 60 which reston the wires of the respective X,Y coordinate grids after the grid wireframe assembly 50 is placed on the top of the fixture 40 and clamped inplace, and after the wires are stretched. As shown in FIG. 5D, eachhold-down bar 60 is formed with a rounded lower surface 62 for bearingagainst the row of parallel wire conductors of either the X or Ycoordinate grid portion of the X, Y coordinate grid or matrix 15 ashereafter more fully appears. The handle 61 permits complete removal ofthe hold-down bars 60 when the grid wire frame assembly 50 is mounted onor removed from the fixture.

A further feature of the grid wire frame assembly 50 should be noted inFIG. 5. Three corners of the grid wire frame 50 are provided withthreaded handles 51 which permit separation of frame sides 50a and 50bfrom sides 50c and 50d (see FIG. 7B). When the grid wire frame assemblyis mounted on the fixture, each of the sides 50a through 50d is clampedto bearing surfaces or pull-out bars of the fixture. Sides 50c and 50dare clamped by clamps not shown to bearing surfaces 42c and 42d of thefixture which are stationary. Thus, grid wire frame assembly sides 50cand 50d remain stationary relative to the fixture. On the other hand,grid wire frame sides 50a and 50b are clamped by clamps not shown to thepull-out bars 45a and 45b which are moveable relative to the fixture ashereafter described in more detail with reference to FIG. 6. Thus, oncethe corner threaded handles 51 are removed from the grid wire frameassembly, grid wire frame sides 50a and 50b may translate with pull-outbars 45a and 45b away from the fixture framework 40 in order to tensionthe X and Y coordinate grids beyond the yield point of the ductile wirecomprising the grids. The grid wire frame sides 50a and 50b maythereafter be reassembled with the other adjacent sides secured to thestationary portions of the fixture for subsequent use. As hereafter willalso more fully appear, the X,Y coordinate grid wires are strung aroundrollers 52 accurately and sequentially spaced along the face of frame 50so that the X,Y coordinate grid or matrix extends over and beyondplatform 55 on which is supported the glass substrate and uncured resinlayer, and furthermore, so that the X and Y coordinate grid elementsextend over and beyond the wire locating rails 56 and below and beyondthe hold-down bars 60 to the peripheral sides of the grid wire frame 50.For example, for a tablet 64" long, wire lengths of 85" length areassembled on the grid wire frame rollers.

Referring now to FIG. 6, the lift platform 55 can be seen mounted withinthe fixture framework 40 at one side on hydraulic lift cylinders 64 and65, in turn, mounted on the fixture beam 44. In this illustration, thelift platform 55 upon which is mounted the glass substrate 12 is inraised position. In this position, prior to placement of a grid wireframe assembly, the glass substrate is prepared by forming a thin resinlayer across the surface of the glass substrate. The hydraulic cylinders64 and 65 have sufficient stroke, for example, three inches (7.6 cm) andone inch (2.54 cm) to lower the prepared glass substrate out of the wayduring mounting of the grid wire frame assembly. In the lower position,the resin layer formed across the surface of the glass substrate ispermitted sufficient time, for example, 15 to 30 minutes, for settlingunder gravity and surface tension. Settling of the surface of the resinlayer may be facilitated by spraying a volatile solvent such as alcoholacross the surface to facilitate release of bubbles and settling.

In preparing the glass substrate with a layer of uncured resin, careshould be taken to use the "clean side" of the float glass. The "cleanside" of the float glass may be identified by methods known in the artand the resin layer should be applied on this side.

The uncured resin, ultra-violet and anaerobic sensitive, may be spreadover the surface of the glass substrate manually be troweling. However,this tends to introduce bubbles which slows the settling and leveling ofthe resin layer. Therefore, the resin is distributed over the surface ofthe glass substrate preferably by pouring or flowing rather than bytroweling. The resin is permitted to settle and level for 15 to 30minutes, achieving a surface thickness of approximately 50 thousands ofan inch. As heretofore described, the resin surface may be sprayed witha volatile solvent mist to facilitate release of bubles and leveling.

Referring again to FIG. 6, one of the pull-out bars 45 is shown on itsmounting of lever arms 71 and 72 typically mounted to rigid arm 73extending from the fixture framework 40. A horizontal hydraulic cylinderassembly 70 is provided adjacent the upper portion of the fixture withpiston 74 mounted to lever arms 71 for forcefully translating thepull-out bar 45 away from or towards the upper surface of the fixture.Hydraulic cylinder 70 is provided, for example, with a four inch (10 cm)stroke for tensioning grid wires of either the X or Y coordinate gridbeyond the yield point and stretching the wires a fixed distancemeasured by the stroke of hydraulic cylinder 70, for example, fourinches (10 cm).

Pull-bar 45 provides an effective extension of the outer bearing surface42 of fixture framework 40 upon which rests the grid wire frame assembly50. As shown in FIG. 6, the parallel wires 18, either the X or Ycoordinate grid component of the grid matrix, pass around rollers 52spaced sequentially along the face of opposite sides of grid wire frameassembly 50. It should be recalled that while two adjacent sides 50a and50b of grid wire frame assembly 50 rest upon pull-out bar extensions 45aand 45b of the fixture framework 40, the other two adjacent sides, 50cand 50d of grid wire frame assembly 50 rest upon and are clamped to thestationary outer bearing surfaces 42c and 42d of the fixture 40.

After the respective sides of the grid wire frame assembly 50 areclamped to the respective pull-out bars or outer bearing surfaces offixture 40, the lengths of wire in the X coordinate direction and the Ycoordinate direction are tensioned to the yield point and then stretchedbeyond that a predetermined distance according to the stroke ofhydraulic cylinder 70. For lengths of wire 85 inches, stretching beyondthe yield point of a distance of four inches has been found satisfactoryto remove all kinks and assure straightness of the wires without damageor weakening.

With 28 gauge tinned copper wire, 8 pounds of force are required on eachstrand or length of wire to achieve the tensioning beyond the yieldpoint and stretching by plastic flow the specified distance. For thelarge size tablet, 64 inches (162.6 cm) in the long direction, a totalforce of over 2,500 pounds (over 1,137 kg) is required for 320 lengthsof wire. Because of the forces involved bearing across the fixture,bolting to the cement floor by means of bolts 78 is desirable. Suchpermanent attachment to the structural ground also facilitates levelingof the lift platform 55 so that the glass plate substrate 12 may bemaintained in a precisely level condition during preparation andsettling of the resin layer formed across the surface of glass plate 12.

A feature and advantage of the use of ductile material wire inaccordance with the present invention is that the individual lengths ofwire yield at about 8 pounds of force, while conventionally used tensilesteel piano wire requires at least 250 pounds of force to begin tostraighten out. While the fixture, according to the present invention,must bear considerable cumulative forces, summed over all of the wires,such forces are still orders of magnitude less and therefore safer thanwould be required in using conventional music wire.

With lift platform 55 and hydraulic cylinder 64 and 65 in the lowerposition, the grid wire frame assembly 50 on which the X, Y grid ormatrix 15 has been assembled, is mounted on the upper surfaces of thefixture 40. The sides of the grid wire frame 50 are clamped to therespective sides of fixture framework 40 and pull-out bars 45. Thehydraulic cylinders 70 are extended the specified stroke distance,stretching and straightening the X and Y coordinate grids 16 and 18.

For better visualization of the grid wire frame assembly 50 and methodset forth above, reference is made to FIGS. 7, 7A and 8. The grid wireframe assembly 50 is shown alone in FIG. 7, including frame sides 50a,50b, 50c and 50d as previously indicated with reference to FIG. 5. Theface of each of the sides supports a row of spaced rollers 52 only a fewof which are shown on each side of the grid wire frame 50. The roller 52is shown in more detail in FIG. 7A and is generally mounted on ashoulder bolt for free rotation in response to movement of the wiresaround the grooves 53 of the respective rollers 52. A feature andadvantage of the invention is that each of the X or Y coordinate gridelements 16 and 18 is formed from a single strand 1 of wire by threadingor stringing back and forth around the rollers 52 on opposite sides ofthe frame 50.

Thus, each of the X and Y grid coordinate elements comprises a single,continuous filament advantageous for continuity testing at an earlystage in the fabrication process as hereinafter described. As previouslyindicated, threaded handles 51 permit disassembling two sides of thegrid wire frame 50 for tensioning the grid wires mounted on the fixturein the manner heretofore described. Another feature and advantage of thesingle filament roller mounting of each of the X and Y coordinate gridsof conducting wire is that the tensioning force is distributed evenlyover all of the lengths of the grid and all of the lengths are equallyand simultaneously tensioned to the yield point and then stretched anequal amount according to the pre-set stroke of hydraulic cylinder 70.

After tensioning and stretching of the X, Y coordinate grids, thehold-down bars or wire clamps 60 are lowered by the threaded handleclamps bearing against the grid wires and urging into the grooves ofwire locating rails 56 as shown in FIG. 6.

The hold-down bars apply and urge the wires into the grooves of the wirelocating rails with slight additional tension. Consideration must begiven to the depth to which the hold-down bars should be clamped inpushing the wires into the grooves. On the one hand, the hold-down barsmust push the wires until they are seated into the grooves and will notlift up. On the other hand, the angle of down tensioning cannot be toogreat as this may tend to lift the wire from the groove on the sideopposite the hold-down bar. The hold-down bars serve to add a slightadditional tension to the wires which are now stretched beyond the yieldpoint, securing them in the desired accurate position over the glasssubstrate and uncured resin layer. Thus, during stretching the wires areslightly above the grooves, and once stretched, they are held down inthe grooves with slight additional tension by the hold-down bars.

The various movements involved are shown in the diagram of FIG. 8 whererelative to lift platform 55 on which is mounted the glass substrate 12prepared with a layer of uncured resin, the pull-out bars 45 moveoutward after which the hold-down bars 60 move downward, urging the gridwires into the grooves 58 of wire locating rails 56 shown in furtherdetail in the diagrammatic view of FIG. 8A.

A check is made to assure that the grid wires are seated appropriatelyin respective grooves 58 of the wire locating rails 56 and to assurethat they are uniformly recessed below the upper surface of the locatingand spacing rail 56. With the wires properly tensioned and stretched,and accurately spaced and aligned within the grooves, immersion of theX,Y coordinate grid or matrix by 70 previously prepared resin layer mayproceed.

Lift platform 55 on which is mounted the prepared glass substrate is atthat time in the lower position. The uncured resin layer formed over thesurface of the glass substrate, supported on lift platform 55, hassettled for 15 to 30 minutes for uniform flatness under gravity andsurface tension. The lift platform 55 is then raised until the uncuredresin layer supported over the glass substrate contacts and immerses theX, Y coordinate grid or matrix 15 supported by the grid wire frameassembly 50 and fixture 40 over the lift platform 55. A stop mechanismor stop switch assures that the surface of the glass substrate stopsbefore contact with the grid wires and, in fact, a distance of 10 to 11thousands of an inch (0.25 mm to 0.27 mm) below the grid wires. In thismanner, both axes of the X and Y coordinate grid or matrix are immersedin the 50 thousands inch (0.12 cm) thick resin layer, both axes orcoordinates spaced from each other and spaced from the upper surface ofthe resin layer and the glass surface below.

After immersion of the X, Y coordinate grid or matrix 15 in the resinlayer 14 previously prepared on the glass substrate 12, the uncuredresin layer is permitted to settle once again for several minutes. Avolatile solvent may again be sprayed over the resin surface to releasebubbles and facilitate settling. Rapid curing of the resin layer isthereafter initiated. Such curing must take place as quickly as possibleto prevent runoff of resin over the edge of the glass substrate becausethe wire volume raises the level of the resin slightly. It should bekept in mind that according to the invention, the resin layer issupported on the glass substrate only under the forces of gravity,surface tension, etc.

In order to achieve a rapid cure, for example, within a curing period offive minutes, a resin or resin mixture is selected sensitive toultra-violet irradiation and anaerobic conditions for acceleratedcuring. Such resins include the acrylics and epoxies.

The rapid cure of the resin layer 14 is accelerated by the use of anultra-violet light hood 80 illustrated in FIGS. 9 and 9A, mounted overthe fixture for raising and lowering over the upper surface of thefixture on which is supported the glass substrate 12, resin layer 14,and X, Y coordinate grid or matrix now immersed in the resin layer. Thehood 80 is constructed and arranged to have the same overall areadimensions as the upper surface of the fixture for enclosing the uppersurface, including the glass substrate and resin layer within the hoodhousing. Thus, the opaque upper surface 81 of hood 80 has the same areaas the upper surface of the fixture and the grid wire fram assembly. Thesidewalls 82 further enclose the volume covered by the hood. The hoodincludes a plurality of ultra-violet light lamps or sources 83 and powersupply ballast 84 within a channel 85 secured to the top of the hood.

A feature and advantage of the ultra-violet light hood 80 is that itfunctions at the same time as a manifold for flooding the resin layerwith an anaerobic gas such as nitrogen or carbon dioxide. The combinedaction of the ultra-violet light irradiation and anaerobic environmenteffects a cure of the resin layer within five minutes. Because thecuring of the resin layer is an exothermic reaction, the flooding withnitrogen or carbon dioxide or other anaerobic gas not only displacesoxygen for rapid curing, but also acts to cool the reaction taking placeacross the resin layer.

Curing of the resin layer with ultra-violet irradiation actually takesplace in a series of steps or exposures. According to one method, four20 to 30 second exposures are used, separated by intervals of, forexample, 30 seconds, followed by a longer exposure of 1 to 2 minutes.Flooding with nitrogen is initiated before the ultra-violet irradiationexposures begin and carries on towards the end. The flooding nitrogennot only provides an anaerobic environment but also acts as a coolingagent for the exothermic reaction. When the curing cycle is completed,the supply of nitrogen or other anaerobic gas is turned off and theultra-violet light fixture or hood is raised for inspection of thetablet.

Upon completion of curing the hood 80 is raised to an upper positionabove the fixture by a standard mechanism (not shown). The hold-downbars 60 are lifted and removed and the tension applied by pull-out bars45, and hydraulic cylinder 70 is released. The grid wire loops extendingfrom the now cured tablet are removed over the rollers 52 for continuitytesting prior to cutting and trimming.

A feature and advantage of the present invention and method andarrangement for assembling the X, Y coordinate grid or matrix 15 on thegrid wire frame assembly 50 is that continuity testing of the grid wiresnow embedded in the resin layer may be completed at an early stage inthe manufacturing process. After continuity testing of each coordinatewire to assure freedom from electrical fault, the wires are trimmed inthe manner shown, for example, FIGS. 1 and 2. The basic glass substrateand resin layer tablet structure may then be incorporated in a laminartablet assembly, for example, of the type described above with referenceto FIG. 3B.

For this purpose, the tablet is placed on a vacuum table and adhesive isspread over the surface of the glass. The honeycomb layer is then bondedto the glass plate. A large, impervious plastic cover, for example, avinyl sheet, is held over the vacuum table by, for example, an aluminumframe, and the vacuum is applied to facilitate pressing and bonding. Thehoneycomb is, in turn, glued to an aluminum base as heretofore describedwith reference to FIG. 3. The thin Formica or other Formica typematerial working surface is bonded to the resin layer also using avacuum to provide a press between the laminar layers, and the assemblyis removed from the vacuum table.

Instead of using an opaque Formica like material for the working surfaceof the tablet assembly, a translucent or transparent material such as avinyl working surface may be used where the tablet assembly is to beback-lighted. In that event, the opaque steel sheet backing andhoneycomb layer are omitted to permit back-lighting to pass through theworking surface of the tablet.

While the invention has been described with reference to particularexample embodiments, it will be appreciated that it is intended to coverall variations and equivalents within the scope of the following claims.

I claim:
 1. A method of fabricating a precision wire grid glass tabletfor translating graphic location or movement information intocorresponding electrical signals comprising:forming a first wire grid bystringing wire of ductile material on a frame assembly and aligning andspacing lengths of wire in a parallel array in a common plane; forming asecond wire grid by stringing wire of ductile material on said frameassembly in a second parallel array and second common plane spaced fromthe first, said second grid comprising parallel lengths of wireperpendicular to the lengths of wire of the first grid; preparing a flatglass plate by spreading a relatively thin layer of resin material overthe flat upper surface of the glass plate; delaying curing of the resinlayer during a period of time permitting the resin layer to distributeevenly and become at least partially settled under the influence ofgravity and surface tension; positioning the frame assembly of first andsecond wire grids over the glass plate; tensioning the lengths of wireof the first and second grids to beyond the yield point of the ductilematerial comprising said wire until the lengths of wire exhibit plasticflow thereby assuring straightness of said lengths of wire prior toimmersing said lengths of wire in said resin layer; raising the glassplate beneath the first and second grids and immersing the lengths ofwire of the first and second grids in the resin layer supported on theupper surface of the glass plate following the at least partial settlingof the resin layer under the influence of gravity and surface tension;and rapidly curing the resin layer whereby the first and second gridsare potted in the resin layer and bonded to the structural glass plate.2. The method of claim 1 further comprising the step of rapidly curingthe resin layer by applying ultra-violet light.
 3. The method of claim 2wherein said resin comprises an anaerobic curing resin and furthercomprising the step of rapidly curing the resin layer by flooding thetablet with anaerobic gas displacing oxygen during the curing step. 4.The method of claim 3 wherein said gas is selected from the groupconsisting of nitrogen and carbon dioxide.
 5. The method of claim 1further comprising the step of spraying a volatile solvent over thesurface of the resin layer prior to curing thereby releasing bubblesfrom the surface of the resin layer and enhancing levelling.
 6. Themethod of claim 1 wherein the step of tensioning the wire comprisesevenly distributing the tensioning force over the lengths of wire of theparallel array comprising the respective grid.
 7. The method of claim 1wherein the step of forming a wire grid comprises stringing a singlepiece of wire back and forth in serpentine fashion with turns atopposite sides of the parallel array on a frame assembly.
 8. The methodof claim 7 wherein said frame assembly comprises roller means staggeredalong opposite sides and wherein the step of forming a wire gridcomprises stringing a single wire back and forth in serpentine fashionaround alternate roller means on opposite sides of said frame assembly.9. The method of claim 7 further comprising the step of testing thecontinuity of the single piece of wire before cutting the wire of eachgrid adjacent the turns at opposite sides of the glass plate and resinlayer.
 10. The method of claim 1 further comprising the step ofaccurately spacing and aligning the lengths of wire after tensioning bypushing the wires into accurately spaced grooves of a wire guide rail.11. A method for fabricating wire grid glass digitizing tabletscomprising:arranging and supporting an X,Y coordinate grid ofrespectively parallel lengths of ductile wire; stretching said lengthsof wire beyond the yield point of the ductile material comprising thelengths of wire until the lengths of wire exhibit plastic flow;preparing a glass substrate by forming a relatively thin layer ofuncured resin over the surface of said glass substrate and permittingthe resin layer to at least partially settle and flatten under theforces of gravity and surface tension; raising the glass substrate belowthe X,Y coordinate grid until the wires of said grid are immersed in theresin layer after the resin layer has at least partially settled;permitting the resin layer to fully settle; and rapidly curing saidresin layer.
 12. The method of claim 11 further comprising the step ofspraying the surface of the resin layer during settling with a volatilesolvent to facilitate flattening.
 13. The method of claim 11 furthercomprising the step of accurately spacing and aligning the lengths ofwire of the X,Y coordinate grid.
 14. Apparatus for fabricating wire gridglass digitizing tablets comprising:a base fixture having a four-sidedframework, each side having outer bearing surface means and inner wirelocating rail means, each said wire locating rail means having an upperedge with grooves therein in accurately spaced and aligned positions forreceiving and retaining grid wires of ductile material and accuratelyspacing and aligning said grid wires within the grooves, said basefixture formed with an inner cavity within the four-sided framework;lift platform means mounted for vertical translation within the cavityof the base fixture, and means for raising and lowering the platformmeans in said cavity; the outer bearing surface means of at least twoadjacent sides of the base fixture, comprising wire pulling barsmoveable from said base fixture and means for translating the wirepulling bars of said at least two sides alternately away from and backtowards the base fixture; hold-down bar means and means for mounting thehold-down bar means to the base fixture at the four sides thereofbetween the respective outer bearing surface means and wire locatingrail means, said hold-down bar means and mounting means constructed andarranged for bearing down on "X" and "Y" coordinate grid wires of afour-sided grid wire frame assembly mounted on the respective outerbearing surface means of the four-sided framework of the base fixture,and for urging said wires into respective grooves of the wire locatingrail means for accurate spacing and alignment; and grid wire frameassembly having four sides with a plurality of roller means accuratelyspaced along said sides around the face of the four-sided assembly forreceiving an "X" coordinate grid wire winding alternately back and fortharound the roller means of two opposite sides and a "Y" coordinate gridwire winding alternately back and forth around the roller means of theother two opposite sides and spaced from the "X" coordinate grid wirewinding, the outer bearing surface means of the base fixture four-sidedframework constructed and arranged to receive the four sides of the gridwire frame assembly, and means for aligning and clamping the grid wireframe assembly to the outer bearing surface means of the base fixturefour-sided framework, said grid wire frame assembly having at least twosides separable from said assembly, said two separable sides orientedfor clamping to the moveable bearing surface means comprising wirepulling bars whereby "X" and "Y" coordinate grid wire windings may betensioned beyond the yield point of said wires with tension evenlydistributed by said roller means and so that the wires exhibit plasticflow.
 15. The apparatus of claim 14 wherein said means for raising andlowering the platform means comprises hydraulic lift means and stopmeans for stopping the platform means at a specified elevation relativeto grid wires accurately spaced and aligned in the wire locating railmeans.
 16. The apparatus of claim 14 wherein said inner wire locatingrail means comprise:first and second wire locating rails for accuratelyspacing and aligning "X" coordinate grid wires in a first plane, saidwire locating rail means further comprising second and third wirelocating rails for accurately spacing and aligning "Y" coordinate gridwires in a second plane spaced from the first.
 17. The apparatus ofclaim 14 wherein said means for translating the wire pulling barscomprises hydraulic cylinder means.
 18. The apparatus of claim 14further comprising ultra-violet light irradiation means and overheadhood means housing said irradiation means, said hood means mounted overthe base fixture and grid wire frame assembly mounted on the fixture,said hood means moveable between a raised position and a lower positionadjacent to the base fixture four-sided framework and a grid wire frameassembly mounted thereon, said hood means constructed and arranged withfour sides for substantially covering the base fixture four-sidedframework and a grid wire frame assembly mounted thereon when the hoodmeans is in the lower position.
 19. The apparatus of claim 18 whereinsaid hood means further comprises gas manifold means for releasinganaerobic gas for flooding the base fixture and grid wire frame assemblysupported thereon with said gas for displacing oxygen.
 20. The apparatusof claim 19 wherein said gas is selected from the group consisting ofnitrogen and carbon dioxide.
 21. The apparatus of claim 14 wherein saidlift platform means is constructed and arranged for raising a glassplate substrate having an uncured layer of resin spread across the uppersurface thereof and wherein said means for raising and lowering theplatform means in said cavity comprises stop means for stopping the liftplatform when the "X" and "Y" coordinate grids are immersed in theuncured resin layer spread across the upper surface of a glass platesupported on the lift platform means.
 22. Apparatus for fabricating wiregrid glass digitizing tablets comprising:base fixture means having afour-sided framework for receiving and retaining a grid wire frameassembly, said base fixture means formed with a central cavity, liftplatform means mounted for vertical translation within the cavity, andmeans for raising and lowering said platform; a grid wire frame assemblyhaving four sides for supporting an X,Y coordinate grid of lengths ofwire of ductile material, and means for clamping said grid wire frameassembly to the base fixture means four-sided framework, at least twosides of said respective grid wire frame assembly and base fixture meansfour-sided framework being separable and moveable from the respectiveassembly and fixture means for tensioning the lengths of wire of an X,Ycoordinate grid at least to the yield point of the wire so that thelengths of wire exhibit plastic flow; said lift platform meansconstructed and arranged for receiving and supporting a glass substrate,having a relatively thin uncured resin layer formed thereon and forraising said glass substrate to immerse the lengths of wire of the X,Ycoordinate grid in said resin layer after said resin layer has at leastpartially settled under the influence of gravity and surface tension.23. The apparatus of claim 22 wherein said base fixture means furthercomprises wire guide rail means having an upper edge with accuratelyspaced grooves formed therein and hold-down bar means for urging lengthsof wire of an X,Y coordinate grid into the grooves of the wire guiderail for accurate spacing and aligning.